By Cameron Chai
National Institute of Standards and Technology (NIST) researchers have revealed that thick arrays of very long carbon nanotubes have absorption capabilities at long wavelengths, which can be used to coat prototype detectors in order to measure terahertz laser power.
The research is aimed at establishing reference standards to calibrate lasers that work at wavelengths ranging from 100 ìm to 1 mm. John Lehman, NIST project leader, says that the coating could be suitable for laser power detectors operating in the terahertz range.
Cupcakes" of vertically aligned carbon nanotube arrays (VANTAs) grown on silicon
The coating known as a vertically aligned carbon nanotube array, VANTA, offers various useful features. VANTA can be handled easily. It comprises of nanotubes, with a length of few micrometers to over a millimeter, to form a thick layer. The VANTA can be seen even without a microscope and can be easily transported from a surface of silicon to a laser power detector.
The coating is extremely dark. The NIST team tested three VANTA samples with average lengths ranging from 40 and 150 ìm to 1.5 mm. They determined that longer nanotubes will reflect relatively less light.
The sample, measuring 1.5 mm long, reflects only 1 % at a wavelength of 394 ìm. Measuring VANTA's reflectance at the terahertz wavelength reveals that laser light can be completely absorbed to deliver precise laser power measurements. The 1.5 mm VANTA has high rates of absorption than gold black coatings. More effort is required to determine uncertainties and parameters such as light angle.
The researchers discovered that the material is capable of absorbing and emitting heat rapidly than other black coatings. As a result, the detectors will be quick to respond and to generate signals. A sufficiently dense coating can absorb lights of long wavelengths.
NIST is developing a terahertz laser for making periodical measurements and a detector known as a thermopile determines the power of laser. This detector generates a voltage in a junction of two metals on application of heat. The researchers utilized the VANTA for coating a thermopile.